Improving the image quality of a magnetic resonance imaging system, i.e. improving signal to noise ratio (SNR) and resolution of images, can in principle be achieved by increasing the magnitude of the static magnetic field as well as realizing a faster gradient switching. In order to enhance the SNR, magnetic resonance imaging (MRI) systems using a high static magnetic field have been developed. However, as the strength of the static magnetic field increases, the frequency of the RF transmitting field linearly increases as well.
Increasing the static magnetic field for example from 1.5T to 3T results in a RF transmitting field whose interaction with a human body can no longer be neglected. Interaction between the RF transmitting field and the human body is observed by, for example, the dielectric resonance effect, since the effective wavelength of the RF transmitting field is comparable to or even smaller than the dimension of the human body that is subject to imaging by the MRI system.
Such a strong interaction not only degrades substantially the RF transmitting field homogeneity and thus the imaging quality, but also can cause concerns about the safety because the electric field associated with the RF transmitting magnetic field increases with the inhomogeneity of the RF transmitting field. In the regime of high magnetic fields, the limits of specific absorption rate (SAR), peripheral nerve stimulation (PNS) and acoustic noise may easily be reached.
In order to produce high resolution and high signal to noise ratio images by making use of high magnetic fields, a MRI system has to operate much closer to the limits of SAR, PNS and acoustic noise compared to an imaging system operating in the regime of lower magnetic field strength.
Consequently, more sophisticated SAR and PNS models, hardware calibration, safety features and patient dependent optimizations are required. As a result, the necessity for pre-scan calibrations appreciably increases. When for example a plurality of specific calibrations has to be performed sequentially, the percentage of time spent calibrating versus clinical scanning time would become too high. Consequently scanning sessions dramatically expand in time and increase in complexity for the patient and the operator alike.
The European patent application EPI 220 153 A2 discloses a method and apparatus for providing a “just-in-time” localizer image of an object of interest from which a high resolution image can be based. This method includes prescribing a second image relative to at least one of a default second image, a first image and a representation of a three dimensional volume data set. The method further includes acquiring the second image, displaying the second image, and prescribing a clinically useful image relative to the second image. The second image and the clinically useful image are prescribed, acquired, and displayed within a single graphical prescription environment included in an imaging system.
In particular, the method and apparatus disclosed in EP 1 220 153 A2 makes use of localizer images that permit a region of interest of a subject being imaged to be visualized relatively quickly, such that the operator may get orientated within the three dimensional space of the patient and/or to locate the desired region of interest within the patient to be further imaged. The localizer images also provide a reference frame or image from which location, orientation, and other imaging parameters associated with one or more target images to be subsequently acquired can be prescribed.
EP 1 220 153 A2 mainly focuses on searching or maneuvering of a scan plane within the subject of interest to find a specific region desired to be imaged. This is primarily performed by using precursor images rather than by using localizer images and/or target images. The method and apparatus disclosed therein does not focus on the general enhancement of the quality of high resolution images obtained by an MRI system.
The present invention therefore aims to provide an MRI system, a computer program product and a method for improving the imaging quality of an MRI system as well as facilitating a selection of a region of interest of a body being subject to magnetic resonance imaging.